This invention relates to drilling fluid compositions possessing increased stability at the down-hole temperatures encountered in deep drilling gas and oil recovery operations, e.g., temperatures of at least about 120.degree. C. and above and preferably of at least about 150.degree. C. and above at well depths of 15,000 ft. or more. More particularly, this invention relates to clay-thickened, water-based drilling fluids, containing a controlled release high temperature-stable polymeric thinner or dispersant.
It is the usual practice in well drilling operations to fill the well bore with a drilling fluid, or mud, which is circulated therein. The drilling fluid is generally pumped down through the drill stem, circulated about the drill bit and then returned to the surface through the annular passage between the drill stem and well wall.
As is known from U.S. Pat. No. 3,035,042, drilling fluid performs a number of essential functions in the drilling operation: it serves to cool and lubricate the drill bits, to carry the cuttings to the surface as the drilling fluid is circulated in and out of the well, to support at least part of the weight of the drill pipe and drill bit, to provide a hydrostatic pressure head to prevent caving of the walls of the well bore, to deposit on the surface of the well bore a filter cake which acts as a thin, semi-impervious layer to prevent undue passage therethrough of fluids, and to perform other functions as are well known in the drilling art. It is important that the drilling fluid system should be kept as simple and inexpensive as possible in order to avoid undue expense in the drilling of the well and enhance performance.
As noted in U.S. Pat. No. 4,411,800, one of the primary requisites of a satisfactory drilling fluid is its ability to inhibit the amount of fluid, normally water, which is lost into the porous strata through which the bore hole traverses. The loss of fluid causes the formation and the build-up of a cake deposit which, after a period of time, can cause sticking of the drill pipe and stoppage of the drilling operation. The drilling fluid must, therefore, be of a nature which permits minimum loss into the porous strata. Agents which impart such property are conventionally termed "thinners" or "dispersants".
The drilling flud components must also be stable and functional after subjection to elevated temperature conditions. In addition to the heat generated by the frictional forces of the drill bit, it is well known that the temperatures encountered in the bore hold are substantially above that found at the earth's surface. The deeper the bore hole is, the higher the temperature encountered. Drilling to greater depths has become common in today's quest for discovering new reserves. It is, therefore, desired to produce a composition capable of exhibiting stability and desired funtionality at the elevated temperatures commonly encountered in deep drilling operations. Materials that have been used in the past to control filtration rates of aqueous drilling fluids by plugging, producing cakes or similar methods, have included materials such as pregelatinized starch, sodium carboxylmethylcellulose, sodium polyacrylates and lignites. Each of these materials have certain limitations. For example, lignite becomes ineffective in high salt concentrations.
Acrylic and methacrylic derivatives, such as those which are copolymerized with hydrocarbon substituted styrenes, such as alpha methyl styrene, para methyl styrene, 2,4-dimethyl styrene, and the like, have been utilized in drilling fluids. For example, U.S. Pat. No. 2,718,497 teaches the use of relatively high molecular weight polymers of these materials to control water loss characteristics of aqueous muds and clay dispersions. Additionally, U.S. Pat. No. 2,650,905 teaches the use of water soluble sulfonated polystyrene derivatives for filtration control in water-based drilling fluids.
Acrylic acid derivatives such as copolymers of acrylamide and sodium acrylate derivatives have been frequently used commercially as flocculants for drilling fluids, and are disclosed in U.S. Pat. Nos. 3,558,545 and 3,472,325. Similarly, a copolymer derived from acrylic acid and acrylamide is disclosed in U.S. Pat. No. 3,323,603 as flocculant for aqueous drilling fluids. U.S. Pat. No. 4,455,240 discloses a drilling mud based on an aqueous clay dispersion and a carboxylic functional polyampholyte, i.e., a polymer derived from the copolymerizations of a carboxylic functional monomer to provide anionic monomer groups, a cationic-containing monomer and optionally, a nonionic monomer.
Published British Application No., 2,044,321A discloses a copolymer drilling fluid additive prepared from (1) a meth(acrylamido alkyl sulfonic acid) or alkali metal salt thereof and (2) a (meth)acrylamide or N-alkyl (meth)-acrylamide. The copolymer may be cross-linked with a quaternary ammonium salt.
However, these and similar materials have been generally found to be unstable at the relatively high temperature conditions encountered in deep well drilling.